Heat Pump System and Method For Heating a Fluid

ABSTRACT

This invention relates to a heat pump system and in particular to a heat pump system and method for heating a fluid. According to one aspect of the invention, there is provided a heat pump system for heating a fluid, said system including: an evaporator for extracting heat from a heat source to vaporise a refrigerant; a compressor fluidly connected to said evaporator for compressing said refrigerant vapour; a condenser fluidly connected to said compressor for transferring heat from said compressed refrigerant to said fluid; a main expansion device fluidly connecting said condenser to said evaporator for reducing the temperature of the refrigerant; means for diverting and reducing the temperature of a portion of said refrigerant from said condenser, and means for fluidly injecting said temperature reduced refrigerant portion into said compressor such that said temperature reduced refrigerant portion mixes with said refrigerant vapour at an intermediate pressure and induces at least quasi-two-stage compression of said refrigerant vapour and said refrigerant portion for discharge into said condenser. According to another aspect of the invention, there is provided a method for heating a fluid, said method including the steps of: extracting heat from a heat source to vaporise a refrigerant; compressing said refrigerant vapour to increase its temperature; transferring heat from said compressed refrigerant vapour to said fluid; diverting and reducing the temperature of a portion of said refrigerant after said transferring step; reducing the temperature of said refrigerant; introducing said temperature reduced refrigerant portion during said compressing step such that said temperature reduced refrigerant portion mixes with said refrigerant vapour at an intermediate pressure and induces at least quasi-two-stage compression of said refrigerant vapour and said refrigerant portion, and discharging said compressed refrigerant to transfer heat to said fluid in said transferring step.

FIELD OF THE INVENTION

This present invention relates to a heat pump system and in particular to a heat pump system and method for heating a fluid.

The invention has been developed primarily for use as a heat pump system and a method for water heating in a cold environment or an environment with large variations in ambient temperature and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use.

BACKGROUND OF THE INVENTION

Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.

Sanitary water needs to be heated to a temperature at or above 60° C. Quite often the water for building heating also needs to be heated to this temperature. An air sourced heat pump system has been used for this type of water heating and conventionally uses an air conditioning compressor. However, owing to the narrow operational temperature range of the air conditioning compressor, the conventional heat pump system cannot work in environments with a wide ambient temperature range, such as an environment where it is very hot in summer but very cold in winter. Similarly, the conventional system cannot work where there is a relatively large temperature difference between the water and the heat source. For example, where the ambient temperature is constantly low, such as a cold environment.

An approach to overcome this problem is to use a two-stage compression system, a multi-stage compression system, or a cascade system. However, such systems require two or more compressors, making the heat pump system complicated, expensive, and difficult to make suitable for large variations in ambient temperatures. The compression system also becomes unnecessary when the ambient temperature is warm.

In cold environments, fossil fuel burning boilers are frequently used to heat water with high running costs and adverse effects on the environment.

OBJECTS OF THE INVENTION

It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

It is an object of the invention in its preferred form to provide a heat pump system with a compressor which can perform quasi-two-stage compression to allow operation in cold environments or environments with large variations in ambient temperature and which is simple and inexpensive.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a heat pump system for heating a fluid, said system including:

an evaporator for extracting heat from a heat source to vaporise a refrigerant;

a compressor fluidly connected to said evaporator for compressing said refrigerant vapour;

a condenser fluidly connected to said compressor for transferring heat from said compressed refrigerant to said fluid;

a main expansion device fluidly connecting said condenser to said evaporator for reducing the temperature of the refrigerant;

means for diverting and reducing the temperature of a portion of said refrigerant from said condenser, and

means for fluidly injecting said temperature reduced refrigerant portion into said compressor such that said temperature reduced refrigerant portion mixes with said refrigerant vapour at an intermediate pressure and induces at least quasi-two-stage compression of said refrigerant vapour and said refrigerant portion for discharge into said condenser.

According to another aspect of the invention, there is provided a method for heating a fluid, said method including the steps of:

extracting heat from a heat source to vaporise a refrigerant;

compressing said refrigerant vapour to increase its temperature;

transferring heat from said compressed refrigerant vapour to said fluid;

diverting and reducing the temperature of a portion of said refrigerant after said transferring step;

reducing the temperature of said refrigerant;

introducing said temperature reduced refrigerant portion during said compressing step such that said temperature reduced refrigerant portion mixes with said refrigerant vapour at an intermediate pressure and induces at least quasi-two-stage compression of said refrigerant vapour and said refrigerant portion, and

discharging said compressed refrigerant to transfer heat to said fluid in said transferring step.

Preferably, the diverting and temperature reducing means includes an expansion device fluidly connected to the condenser and the compressor. The expansion device preferably includes a capillary tube or an expansion valve. The expansion device may further include a heat exchanger, such as an intercooler.

It is preferred that the diverting and temperature reducing means includes a bypass passage fluidly connecting the condenser and the expansion device.

The fluid injecting means preferably includes a fluid injection valve for controlling the flow of the refrigerant portion into the expansion device. The compressor preferably includes a fluid injection port connected to the fluid injection means. It is preferred that the fluid injection means includes a check valve connected to the fluid injection port.

The method preferably includes the step of returning said refrigerant from said temperature reducing step to said vaporising step.

The main expansion device is preferably fluidly connected to the condenser by a first pipe. The first pipe is preferably connected to the bypass passage. The main expansion device may be an expansion valve.

The capillary tube is preferably in close proximity with the first pipe to cool the refrigerant passing through the first pipe to the main expansion device. In one preferred form, the capillary tube is helically wound around the first pipe. A downstream end of the capillary tube may be connected to a section of pipe. The pipe section is preferably in contact with the first pipe to transfer heat between the first pipe and the pipe section. The pipe section may lie substantially parallel to the first pipe and may be fixed to the first pipe by metal clamps or other suitable fastening means. Heat transfer paste is preferably interposed between the pipe section and the first pipe to facilitate heat transfer. The pipe section is also preferably deformed to conform to the first pipe.

Where the temperature reducing means includes an expansion device and an intercooler, it is preferred that the intercooler is fluidly connected to the condenser and the main expansion device so that refrigerant passes through the intercooler to the main expansion device and exchanges heat with the refrigerant portion passing through the intercooler.

The fluid that is to be heated is preferably water. The heat source may be ambient air.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing, in which:

FIG. 1 is a schematic diagram of a heat pump system for heating water according to the invention; and

FIG. 2 is a schematic diagram of another embodiment of the invention.

PREFERRED EMBODIMENT OF THE INVENTION

Referring to FIG. 1, the heat pump system for heating water includes an evaporator 1 for vaporising a refrigerant by transferring heat from an ambient heat source 3, a compressor 4 fluidly connected to the evaporator 1 for compressing the refrigerant vapour and a condenser 5 fluidly connected to the compressor 4 for transferring heat from the compressed refrigerant to the water 6. The heat source 3 is the ambient air of a cold environment and the compressor 4 is a conventional compressor for low temperature refrigeration with a liquid injection port 7.

An expansion device 8 in the form of a capillary tube is fluidly connected to the condenser 5 and the compressor 4 to divert a small portion of the condensed refrigerant and reduce its temperature. A fluid injection means 9 fluidly injects the temperature reduced refrigerant portion into the compressor 4 from the capillary tube 8. The temperature reduced refrigerant portion mixes with the refrigerant vapour that has been compressed to an intermediate pressure in the compressor 4 and induces at least quasi-two-stage compression. The combined refrigerant (the refrigerant vapour and the refrigerant portion) is then further compressed and discharged into the condenser 5.

A main expansion valve 10 is fluidly connected to the condenser 5 by a pipe 11 and to the evaporator 1. A bypass passage 12 diverts the refrigerant portion from the pipe 11 to the capillary tube 8.

The fluid injection means 9 includes a fluid injection solenoid valve 13 for turning liquid injection of the refrigerant portion on and off, and a pipe section 15 for delivering the temperature reduced refrigerant portion through a check valve 17 to the injection port 7 at the compressor 4. The check valve 17 ensures that only the temperature reduced refrigerant portion enters the compressor 4 and prevents any backflow of refrigerant from the compressor 4 through pipe section 15 to the capillary tube 8 and the fluid injection solenoid valve 13.

The capillary tube 8 is helically wound around the pipe 11 to cool down the refrigerant passing through the pipe 11 as it flows towards the main expansion valve 10. The pipe section 15 is also fixed to a portion 21 of the pipe 11 by metal clamps and lies substantial parallel to and in contact with the pipe portion 21 to facilitate heat transfer between the pipe section 15 and the pipe portion 21. Heat transfer paste is also applied between the pipe section 15 and the pipe portion 21. The pipe section 15 can be deformed to conform to the pipe portion 21 to improve heat transfer.

Other elements of the heat pump system include a liquid solenoid valve 23 for turning the flow of condensed refrigerant on and off and a filter/drier 25 located between the condenser 5 and the capillary tube 8. A sight glass 27 is provided on the pipe 11 for observing the refrigerant prior to entering the main expansion valve 10. A de-ice solenoid valve 29 is also provided in a subsidiary line 31.

The operation of the heat pump system will now be described. Refrigerant in the evaporator 1 is vaporised using heat extracted from the ambient air 3. The compressor 4 draws the refrigerant vapour from the evaporator 1 and compresses it from a low pressure, low temperature vapour state to a high pressure, high temperature vapour state. The high pressure, high temperature refrigerant vapour is then exhausted to the condenser 5, which acts as a heat exchanger to pass heat from the refrigerant vapour to the water 6. As a result of this process, the refrigerant is condensed into a liquid and subcooled.

The liquid refrigerant then passes through the liquid solenoid valve 23 and filter/drier 25 to remove moisture and contaminants from the refrigerant. After passing through the filter/drier 25, the majority of the liquid refrigerant flows through the pipe 11 to the main expansion valve 10. The liquid refrigerant expands through the main expansion valve 10, causing its pressure and temperature to drop. The temperature of the refrigerant is now below the temperature of the ambient air 3. The refrigerant then enters the evaporator 1 where heat is again transferred from the ambient air 3 to the refrigerant. The vaporised refrigerant is subsequently drawn into the compressor 4 and the cycle repeats.

While most of the liquid refrigerant enters the main expansion valve 10, a small portion of the liquid refrigerant (which may be about 10% of the total refrigerant) enters the bypass passage 12 from the pipe 11 and passes through the liquid injection solenoid valve 13 to capillary tube 8. The capillary tube 8 expands the refrigerant portion, causing its pressure and temperature to drop. The temperature reduced refrigerant portion then passes through the pipe section 15 and the check valve 17 to the injection port 7. The refrigerant portion in liquid/vapour form is then injected to the compressor 4 to mix with, and cool down, the superheated refrigerant vapour in the compressor 4 after quasi-first-stage compression (that is, the refrigerant portion is injected into the compressor after the refrigerant vapour has been compressed to an intermediate pressure). As a result, quasi-second stage compression takes place and the combined refrigerant vapour and refrigerant portion is compressed to a final pressure. The compressed refrigerant is then discharged into condenser 5.

Since the refrigerant in the compressor 4 has undergone at least quasi-first-stage compression, introducing the temperature reduced refrigeration portion into the compressor 4 to mix with the superheated refrigerant reduces the temperature of the refrigerant prior to the next stage of compression and thus reduces the temperature in the compressor for subsequent compressions. This results in the pressure ratio for each stage of compression being reduced to the desired level for quasi-two-stage compression, and thus improves the efficiencies for each compression. Quasi-two-stage compression, combined with intercooling in the compressor due to fluid injection of the refrigerant portion, also reduces the power drawn by the heat pump system (compared to single-stage compression). The fluid injection valve 13 and the check valve 17 controls the timing and direction of the temperature reduced refrigerant portion that is injected into the compressor 4. Thus, at least quasi-two-stage compression can be controllably achieved by a single compressor. Consequently, there is a significant increase in the difference between the condensing temperature and the evaporating temperature, increasing the operating ambient temperature range of the heat pump system.

In its preferred form, the expansion device 8 is a capillary tube to simplify the heat pump system. The capillary tube 8 also permits the temperature reduced refrigerant portion to be used simply after expansion to absorb heat from the liquid refrigerant in the pipe 11 before it enters the main expansion valve 10. As described above, the capillary tube 8 is helically wound around the pipe 11 and the pipe section 15 located substantially parallel to and in contact with the pipe 11. In this way, additional subcooling of the refrigerant in the pipe 11 takes place, which reduces the risk of the liquid refrigerant flashing prior to entry into the main expansion valve 10.

While an approximate amount of 10% of the total refrigerant is diverted to the capillary tube 8 in the embodiment, the amount of the refrigerant portion that is diverted depends on the temperature of the ambient heat source and the water temperature that is required.

While the above description represents a preferred configuration of the invention, it will be appreciated that components of the system can be varied in other embodiments.

A second embodiment is illustrated in FIG. 2, where corresponding features have been given the same reference numerals. In the second embodiment, the expansion device 8 is an expansion valve 33 with an intercooler 35. The intercooler 35 is fluidly connected to the condenser 5 and the main expansion valve 10. The bypass passage 12 is located downstream of the intercooler 35. Liquid refrigerant from the condenser 5 enters the intercooler 35. The intercooler 35 initially cools down the liquid refrigerant before the refrigerant portion is extracted via the bypass passage 12 to the liquid injection valve 13. The refrigerant portion passes through expansion valve 33 to further reduce its temperature and pressure. The temperature reduced refrigerant portion is then returned to the intercooler 35 to exchange heat with the liquid refrigerant from the condenser 5 passing through the intercooler 35 before being delivered to the compressor 4. As in the first embodiment, the refrigerant portion mixes with the refrigerant vapour in the compressor to induce at least quasi-two-stage compression. The liquid refrigerant portion is extracted after the intercooler 35 so that the probability of flashing of the refrigerant in both the expansion valve 33 and the main expansion valve 9 will be reduced.

In other embodiments, multi-stage compression can be induced if required by injecting additional temperature reduced refrigerant portion(s) after quasi-second stage compression.

The compressor may be a refrigeration compressor with one or more liquid injection ports built in, or any compressor modified to be equipped with liquid injection port(s).

The description of the heat pump system has been simplified to assist understanding of the invention. It will be appreciated that there are other parts and control and safety mechanisms in the heat pump system which have been omitted from the description but do not affect the basic operation of the system in its preferred form.

The invention in its preferred form as described above provides an energy efficient and practical system of water heating, particularly for an air sourced heat pump system delivering heat from a cold temperature environment. The invention in its preferred form replaces current fossil fuel burning boilers, thereby reducing any adverse impact on the environment.

Although the invention has been described with reference to a specific example it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. 

1. A heat pump system for heating a fluid, said system including: an evaporator for extracting heat from a heat source to vaporise a refrigerant; a compressor fluidly connected to said evaporator for compressing said refrigerant vapour; a condenser fluidly connected to said compressor for transferring heat from said compressed refrigerant to said fluid; a main expansion device fluidly connecting said condenser to said evaporator for reducing the temperature of the refrigerant; means for diverting and reducing the temperature of a portion of said refrigerant from said condenser, and means for fluidly injecting said temperature reduced refrigerant portion into said compressor such that said temperature reduced refrigerant portion mixes with said refrigerant vapour at an intermediate pressure and induces at least quasi-two-stage compression of said refrigerant vapour and said refrigerant portion for discharge into said condenser.
 2. The heat pump system of claim 1 wherein the diverting and temperature reducing means includes an expansion device fluidly connected to the condenser and the compressor.
 3. The heat pump system of claim 2 wherein the expansion device includes a capillary tube.
 4. The heat pump system of claim 2 wherein the expansion device includes an expansion valve.
 5. The heat pump system of claim 2 wherein the expansion device includes a heat exchanger, such as an intercooler.
 6. The heat pump system of claim 2 wherein the diverting and temperature reducing means includes a bypass passage fluidly connecting the condenser and the expansion device.
 7. The heat pump system of claim 2 wherein the fluid injecting means includes a fluid injection valve for controlling the flow of the refrigerant portion into the expansion device.
 8. The heat pump system of claim 1 wherein the compressor includes a fluid injection port connected to the fluid injection means.
 9. The heat pump system of claim 8 wherein the fluid injection means includes a check valve connected to the fluid injection port.
 10. The heat pump system of claim 1 wherein the main expansion device is fluidly connected to the condenser by a first pipe.
 11. The heat pump system of claim 10 wherein the capillary tube is in close proximity with the first pipe to cool the refrigerant passing through the first pipe to the main expansion device.
 12. The heat pump system of claim 11 wherein the capillary tube is helically wound around the first pipe.
 13. The heat pump system of claim 1 wherein the temperature reducing means includes an expansion device and an intercooler, said intercooler fluidly connected to the condenser and the main expansion device such that refrigerant passes through the intercooler to the main expansion device and exchanges heat with the refrigerant portion passing through the intercooler.
 14. A method for heating a fluid, said method including: extracting heat from a heat source to vaporise a refrigerant; compressing said refrigerant vapour to increase its temperature; transferring heat from said compressed refrigerant vapour to said fluid; diverting and reducing the temperature of a portion of said refrigerant after said transferring; reducing the temperature of said refrigerant; introducing said temperature reduced refrigerant portion during said compressing such that said temperature reduced refrigerant portion mixes with said refrigerant vapour at an intermediate pressure and induces at least quasi-two-stage compression of said refrigerant vapour and said refrigerant portion, and discharging said compressed refrigerant to transfer heat to said fluid in said transferring.
 15. The heat pump system of claim 14 wherein said method includes the step of returning said refrigerant from said temperature reducing step to said vaporising step.
 16. The heat pump system of claim 14 wherein fluid to be heated is water.
 17. The method of claim 14 wherein the heat source is ambient air. 